// includes -> boolean
// const arr = ['es6','es7','es8']
// console.log(arr.includes('es7'));
// console.log(arr.includes('es9'));
// console.log(arr.includes('es6', 1)); // 从索引为1的地方开始找
// console.log(arr.includes('es7',-2));  // 从倒数第二个开始找

// console.log(arr.indexOf('es7'));   // 1 
// console.log(arr.indexOf('es7') > -1);  // true

const arr = ['es6',['es7','es8'],'es9', NaN, 2]
console.log(arr.includes(['es7', 'es8'])); // false 
console.log(arr.indexOf(['es7', 'es8'])) // -1
console.log(arr.includes(NaN)); // true
console.log(arr.indexOf(NaN)) // -1